Energy balance of dairy cattle in relation to milk production variables and fertility.

Variables derived from milk yield records were investigated to find an easy to measure and readily available indicator of the energy balance status of a lactating cow. Weekly energy balances during the first 180 d in milk (DIM) were calculated from weekly yield, live weight, and energy intake records for 470 first lactation heifers. The energy balance curve for each cow was estimated using a random regression model. From each curve, three measures were calculated to describe the energy balance status: 1) total energy deficit in early lactation, 2) interval for return to positive energy balance, and 3) lowest value (nadir) for energy balance. Mean energy deficit per lactation was 776.8 MJ of NE(L)/d, interval for return to positive energy balance was 41.47 d, and nadir was -33.72 MJ of NE(L)/d. Regression analysis to relate these variables to interval to start of luteal activity (measured using progesterone profiles) showed that a low nadir of energy balance was related to delayed resumption of luteal activity. In general, a 10 MJ of NE(L)/d lower nadir of energy balance corresponded to a delay of ovulation of 1.25 d. A relatively strong decrease in fat percentage during early lactation was significantly correlated with lower nadir of energy balance, larger energy deficit, and later return to positive energy balance. The maximal correlation was between nadir of energy balance and a decrease of milk fat percentage. This correlation remained above 0.60 throughout the first 26 DIM but dropped to 0.14 at 180 DIM. Large decreases in milk fat percentage were related to high initial fat percentages at the start of lactation and slightly lower fat percentages later during lactation. Hence, we concluded that a decrease in fat percentage during early lactation might serve as an indicator of energy balance.